Cleaning of gases



March 5, 1940;

E. ANDERSON 2,192,172

CLEANING 0F GASES Filed June 1, 1938 INV ENT OR. [Va/a An derso BY Eat, flaw-f5 A ORNEYS Patented Mar. 5, 1940 v e v g UNITED STATES PATENT OFFICE CLEANING OF GASES Evald Anderson, San Marino, Calif., assignor to Western Precipitation Corporation, Los Angeles, Calif., a corporation of California Application June 1, 1938, Serial No. 211,306

Claims. (01. 1837) This invention relates broadly to cleaning at a Sign opposite to the sign of the charges upon c s, a more specifically to an apparatus for the particles, and acts as a collecting electrode. electrically precipitating suspended particles In the precipitating field, the charged particles in from gases, comprising a two-stage precipitator the gas migrate to the surface of the collecting 5 including a collecting electrode which may be electrode, Where t y give p their C 5 kept covered with a liquid film. Th inv nti come neutralized, and adhere-to the surface of also includes a method for precipitating suspendsaid electrode. From this electrode the pr ed particles from a stream of gas. tated material may be removed by rapping the In the following description and in the appendelectrode so as to cause the adhering material to ed claims, the term discharge electrode will be fall away from the collecting surface into a 001- 10 understood to designate an electrode that facililecting box from Which it y he removed i tates corona discharge therefrom, because it has e to time- It is necessary to divert discona configuration that establishes sufficiently high tinue passing the stream of gas to be treated potential gradient at or near its surface to create during t e Clean-Out p o Which 000111 more corona discharge before there is a disruptive disor less freque y depending p the q y 15 charge or spark-over. For this purpose th 11 of material removed from the gas. charge electrode usually takes the form of a I the Sc m dt type tw -s preeipitateif, it member of small surface area, such as a sm ll has not been possible for collection efficiency and. diameter wire or a rod provided with sharp edges gas capac ty b to reach simultaneously the or points whereby there may be created in th highest possible values. As was pointed out, the 20 immediate vicinity thereof a sufiiciently high precipitated particles are neutralized at the colelectric field intensity to cause ionization and leeting electrode, which is net Opp y a corona discharge. The term non-discharging charg electrode, d consequently n0 pp electrode will be understood to designate an ciable force tends to hold the precipitated lec ode that minimizes or prevents corona, di terial in continued contact with the collecting 25 charge therefrom because it has a configuration electrode. In fact, some of the precipitated parthat establishes a sufiiciently low field concentratieles p p a Charge from the collecting ctien at o ear the surface to suppress corona. trode and are repelled therefrom because of thedischarge at elevated potentials below the voltage similarity of h charges Since the material required for disruptive discharge or spark-over. heres only very loosely to the collecting elec- 30 For this purpose, a non-discharging electrode trode, some of it is redispersed into the gas stream usually is one of extended surface area, substandue in p rt to erosion of the built-11p Cake y e tially free from sharp corners or other parts of currents of gas p s thereby n Carrying sharp surface curvature at all portions which are away agglomerated masses of Precipitated P 1ocated within the electric field, so as to sube and in P t0 the projection o particles 35 stantially avoid ionization or corona discharge at from the collecting electr into e s stea that electrode. because of electrical forces, in which case the A patent to W. A. Schmidt 1,343,285, granted particl s d p s t on t opposing ec rode. The June 15, 1920, discloses an electrical precipitator redispe se Particles carried y y gas erosion of the two-stage or separated field type. In the are "not again precipitated, but are carried out 40 apparatus shown therein, two spacially separate of the treating apparatus into the zone where fields are used. Inthe first of these electric fields, purified gas is desired- O to c e se fcommonly called the charging field, a silent elecficiency, it is necessary to cut down erosion in trical discharge is maintained between a disthe precipitating chamber; and this is accomcharge electrode and a non-discharging electrode. plished by lowering the gas velocity through the 45 Th gas bearing fine particles of matter to be chamber which results in a severe loss in caprecipitated is passed between the electrodes pacity. Efficiency and capacity are sacrificed where the particles adsorb ions or electrons and ne r e Other; high emeiehey has formerly acquire electrical charges of the same sign as the been obtained at the expense of capacity, or high attached ions or electrons, and consequently of capacity at the expense of efficiency. 50 .the same sign as the discharge electrode. From This disadvantage of the Schmidt type prethe charging field the gas is led through a precipitator is not found in a single stage precipicipitating field between two non-discharging tator, wherein both charging and precipitation electrodes maintained at a high potential difiertake place in the same field, or in a series of like ence. One of these electrodes is kept charged fields. It has been common practice to efiect 55 charging of the suspended particles by passing them between two opposed electrodes between which a high potential difference is maintained,

one of the electrodes being of a type to produce 5 therefrom silent or corona electrical discharge that causes suspended particles to become charged with the same electrical sign as the discharging electrode. ionizing action. The charged particles migrate, under the influence of the electric field between the electrodes, toward the other electrode which is a non-discharging electrode of extended surface, and collect or become precipitated upon the second electrode which is consequently termed the collecting electrode. A precipitator of this type using only two electrodes for the complete operation uses but a single electric field in which both charging and precipitation take place.

In this singleefield type of precipitator the continuous impingement of the corona current on the dust layer on the collecting electrode maintains' an electrical charge'on the precipitated particles so that the particles are held to the collecting electrode by the electrostatic field, and particles which become redispersed in the air stream after precipitation are immediately recharged and reprecipitated upon the collecting electrode; consequently the single stage precipitator allows much higher gas velocities than a two-stage precipitator -of corresponding size.

However, the ordinary single stage precipitator is open to the disadvantage that optimum conditions for charging are not identical with optimum conditions for precipitation, and a compromise must be effected so that neither ionization nor precipitation are carried out under the most favorable conditions.

There has been observed in electric precipitators a phenomenon termed back corona that greatly interferes with proper collection of particles and so reduces the collection eficiency.

Without discussing the various theoretical aspects of back corona, it is sufiicient to say that it appears when collecting solid particles that are relatively poor conductors. After a thin layer of these high resistance particles covers the collecting electrode, the layer usually punctures at some point, and at this point there first occurs a corona discharge fromthe collecting electrode and later, if the potential is sufficient, an arcing I over to the ionizing electrode. The back corona or are prevents proper charging and precipitation of the particles which are allowed to pass on out of the precipitator. The subsequent re- 5 duction in collection efficiency is sometimes very great. Back corona phenomena are characteristic of the single-field precipitator and are not found usually in the precipitating field of the separated field type because the collection. elec- 3 trode is opposed only by another non-discharging electrode; and under ordinary conditions there is much less tendency to form a corona or are even at a puncture in the precipitated dust layer. Consequently, the latter type gives greatly improved performances under certain circumstances, as it is free from back corona in the precipitating field. The separated field precipitator ofiers other marked advantages over the single field precipitator because it can be more compactly builtand uses less current, so that it is more economical to install and operate.

Another advantage of the separated field type of electrical precipitator results from the fact that, for a given spacing between two opposing electrodes, a materially higher potential difierence This is termed charging or may be maintained, without causing disruptive discharge or arcing therebetween, when both electrodes are of the non-discharging type than when one of the electrodes is a discharging elecof charged particles is materially reduced, resulting in an increased efiiciency of collection or. an increased capacity in terms of volume of gas treated per unit time, or both increased efficiency and increased capacity, in an apparatus of a given size. Additional advantages by way of more rapid precipitation are also made possible by the fact that the opposing non-discharging electrodes may be placed much closer together than has been found-practical when one of the electrodes is a discharging electrode, thus decreasing the mean distance through which the charged particles must be moved by the electric field in order to reach, a collecting surface. The quantitative value of these advantages is greatly diminished when a poorly conductive or highly resistant dust is collected. A thin layer of such highly resistant material on the face of the collecting electrode causes a disproportionately large fraction of the total potential drop between the two opposed electrodes to exist across the dust layer. The result is that the effective potential remaining for precipitation is greatly decreased, and a corresponding decrease in collection efiiciency takes place. This situation does not occur when the electrodes are kept continually clean or the dust is highly conducting. The physical conditions causing this are analogous to those creating back corona, but the results are not as detrimental though they are definitely undesirable.

Electric precipitators of all types have been used chiefly for industrial purposes, and in this field it has been generally immaterial whether or not any new compounds or gases have been formed by the action of the electric field. ,But with the present trend toward air conditioned offices and homes electrical precipitating units are being designed to clean air used for ventilation; and in this type of use it is of prime importance that the I precipitator not produce any substances that are either annoying because of their odor or dangerous to health. Consequently, production of ozone and oxides of nitrogen, formerly of little significance in industrial installations, has now become a factor of importance in air conditioning units, since these gases are distinctly irritating to many people, and ozone, according to some medical authorities, is potentially harmful in excessive concentrations. What upper limit of ozone concentration is permissible is not definitely established though it is now set at various points approximately averaging one part per ten million parts of air, a concentration that is only a small fraction of that produced by most conventional precipitators.

Thus it becomes a general object of my invention to provide an improved separatedfield type of electric precipitator in which highervelocities can be used than heretofore, thus securing greater capacity for a treater of a given size betrapped against blowing out. of the treater.

cause the precipitated particles are effectively A further object is to provide a precipitator in which conditions that reduce the effective voltage of the precipitation field are prevented from arising, thus keeping the precipitator always at a relatively high precipitation efficiency.

An additional object of my invention is to provide an electrical precipitator that does not introduce into the air physiologically objectionable products in quantities above established tolerance limits, so that the precipitator may be adapted to clean air for ventilating purposes.

These and other objects which will be pointed out hereinafter are attained according to my invention by passing a stream of gas containing suspended particles to be precipitated through successive electric fields. In one of these fields the particles are electrically charged, and in another of the fields substantially all the charged particles entering the field are precipitated and simultaneously trapped against redispersion into the gas stream, and continually removed from the collecting surface.

In apparatus for carrying out my novel method of electrically precipitating suspended particles, I provide two successive spacially separate electrode arrangements, each of which is adapted to maintain an electric field, the first of these fields being an ionizing or charging field, and the second being a substantially non-discharging precipitating field. In the second of these fields redispersion of particles into the gas stream after precipitation is successfully prevented by maintaining the collecting electrode wetted with a film of suitable liquid, for example, water, and preferably keeping this surface film of fluid in uniform unidirectional motion so as to continuously wash the collecting electrode free of precipitated material.

The attainment of the above objects and advantages of this invention, as well as others which will appear hereinafter, will be more particularly described and pointed out with reference to the accompanying drawing in which:

Fig. 1 is a vertical sectional view, partly in perspective, of a unit of one form of apparatus for carrying out the method of this invention;

Fig. 2 is a horizontal section along the plane 2-2 of Fig. 1;

Fig. 3 is a horizontal section along the plane 3-3 of Fig. 1;

Fig. 4 is a modified form of apparatus for accomplishing the objects of this invention; and

Fig. 5 is a horizontal section, partly in plan, along the plane 5-5 of Fig. 4.

In Figs. 1, 2 and 3, there is shown a tubular electrical precipitating unit in which the tube is divided into two sections I and 2 which are interconnected electrically through the medium of the liquid sump 3. These parts are preferably made of sheet metal or like conducting material,

since the parts may conveniently be joined by welding. With a water film, non-conducting material may be used if, desired.- At the bottom of tube I is an inlet header 4 fitted with an inlet pipe 5. The top of tube 2 is flared outwardly and rises into a liquid feeding basin designated by the numeral 6, the top of said basin being slightly above the top of the flared portion of tube 2. The feeding basin is supplied with water, or other suitable liquid, through a pipe I. Containing the feeding basin and surrounding the tube 2 is a gas receiving means comprising a header 8, similar to the inlet header 4 above referred to. A gas outlet pipe 9 welded to the receiving header leads the treated gas to the locaers, liquid feeding basin, sump and cooperating pipes are joined in any suitable manner, but preferably are welded together for mechanical strength.

Located centrally and coaxially of. tubes I and 2 is the rigid central. electrode generally indicated at I0. Electrode I is mounted at its upper end in insulator bushing II which passes through'the top wall of receiving header 8, the opening beingsomewhat larger in diameter than the bushing. The centralelectrode and bushing are mounted on the top wall of the receiving header in a flanged ring I 2 which rests upon a flexible gasket I3 supported by the top wall. Suitable bolts I30.

hold the electrode mounting assembly in position. The electrode assembly may be moved bodily in a direction radial of the tube to the extent oi the clearance between bushing II and the top wall of the header, and the electrode may be adjusted angularly with respect to the tube axis by tightening bolts I4 at one side of the bushing more than at the opposite side so as to com press gasket I3 unequally.

The electrode assembly I0 may have any one of a number of different forms but preferably comprises a length of. rod I4 at the lower end of which is attached a-short length of smooth steel wire I of smaller diameter serving as a discharge electrode The rod I4 commencing just above the point of attachment of the small steel wire I5,

is surrounded by a smooth-walled cylindrical jacket or shield I6 of larger diameter than the rod, said shield being secured to or supported on the rod I4 in any suitable manner, as by welding, and forming in effect an integral part of the electrode. The shield I6 preferably is formed as a hollow tube for the sake of lightness in weight, and is closed at both ends, preferably by rounded end portions IIia. As will be well understood by persons skilled in the art, electrode sections having exposed surfaces of relatively small radius of curvature, such as is provided in this case by the steel wire I5, bring about silent or corona .discharge when a high potential is applied to the central electrode. However, shielded section I6 has no such outer surface of small radius and consequently there is substantially no corona discharge from this portion of the electrode. Thus, the parts I, 2 and I6 constitute non-discharging electrodes. When using flexible electrodes, such as wire, it has been necessary toat- A tach them at their lower ends to weights or ing thereof. The steel wire section I5 of the electrode assembly extends below the level of the 1 flanged top .of tube section I, and in cooperation with tube section I as non-discharging complementary electrode, comprises the charging or ionizing section of the apparatus. The shielded portion I6 of the electrode in cooperation with tube section 2 comprises the precipitation and collecting section of. the apparatus, and the shield I6 extends throughout at least the major portion of the height of said tube section.

A high electrical potential difference is applied between the central electrode assembly and the tube comprising sections I and 2, preferably by connecting the central electrode assembly to the negative terminal of a source of unidirectional current, not shown, such as a transformer and rectifier assembly familiar to those skilled in the art, the positive terminal of the current source either being directly connected with the tube or indirectly so connected through ground. The current source is adapted to create a potential between the electrodes sufiiciently high to cause corona or silent discharge from the discharge electrode l5 but not high enough to cause arcing or disruptive discharge. The same potential difference establishes and maintains a substantially static precipitating electrical field between nondischarging electrode sections it and 2. A potential difierence of the order of 25 kilovolts times the number of inches between the electrodes in the precipitating zone has been found to be advantageous in many cases.

In operation, gas bearing suspended matter to be precipitated enters the inlet header 4 through the pipe 5 from a source, not shown, of gas to be cleaned. The gas flows upwardly from the header 4 into the ionizing zone in tube section l where it is acted upon by the corona discharge tocharge the particles of suspended matter with an electrical sign like that upon the discharge electrode IS. The ionizing zone is preferably made as short as possible and the gas velocity is kept as high as possible so that no excess current above the minimum required for substantially complete charging of suspended particles is used. In this way the most efiicient charging conditions are maintained. From the charging zone, the gas fiows through the precipitation zone within tube section 2 where the previously charged particles migrate to the walls of the tube and are precipitated thereupon under the influence of the electrostatic field maintained between electrodes is and 2 The length of the precipitation zone depends on the type of. particles collected and their rate of travel toward the collecting electrode surface, but is comparatively short because substantially all the suspended charged particles are precipitated in the minimum length of chamber under optimum conditions of precipitation. In some cases it may be desirable to increase the length of the charging zone or the precipitation zone in. order to take care ofoverloading conditions or the like.

In general, the charging zone may be substantially shorter than the precipitating zone and may advantageously have a length of the order of one-tenth the length of the precipitating zone. From the precipitation zone the treated gas enters receiving header 8 and thence by way of pipe 9 is led to the point of delivery.

, v During precipitation, the inner walls of tube 2 are covered continuously with a film of liquid which fiows from top to bottom. The electric precipitating field is maintained between the electrode member l6 and this continuous film of liq- .uid, and thus may be described as terminating upon the liquid film. The liquid enters thefeeding basin 6, through pipe 'l'from a source, not shown. The liquid rises within the basin to the lip of the flared portion at the top of tube 2 whence it overfiows uniformly along the periphery into the tube, continuously flowing down the inside "of the tube in an unbroken smooth film. As particles are precipitated on the collecting electrode, the moving film of water entraps them, carrying them downward and maintaining the collecting electrode always in a clean condition. The bottom of tube 2 is flared out to'for'm a belllike portion which leads the liquid bearing precipitated matter into a quiescent zone in thesump' 3 where it is collected and drawn 011 to a reservoir, not shown, through pipe II. The liquid used where the precipitated particles are not readily wetted by water, or an aqueous or other liquid solvent may be used where it is desired to charged oppositely, that is, of opposite sign or polarity, with respect to the discharge electrode member'li and the charged particles entering the precipitating field, and that the direction of the precipitating field is such as to cause precipitation of such charged particles upon the liquid film on electrode member 2.

A modification of the above-described apparatus is shown in Figs. 4 and 5. The only essential diiference between the the two treaters is that in the modified form the extended surface electrode of the ionizing zone and the collecting electrode of the precipitation zone comprise one continuous tube l8, while in the previously described apparatus these two electrodes, although spacially located in corresponding positions, are physically separate and distinct. Due to the construction of the modified apparatus, the liquid dissolve some constituent of the precipitated mafilm fiows not only upon the surface of the col-- lecting electrode, but also upon the extended surface electrode of the charging zone. intermediate the charging and the precipitation zones is eliminated, and the inlet header l9, provided with a drain pipe 20,'performs the added function of a sump. It is apparent that this construction is considerably cheaper to make than the construction of Fig. 1, since the straight tube 18 replaces the tube sections I and 2. The central electrode assembly 2| is the same as the corresponding electrode assembly III in Fig. 1, and is adjustably mounted in similar fashion. The headers I9 and 22 and their inlet and outlet The sump pipes, the feed-basin 23 and its supply pipe, and

the electrical fittings and connections functionally correspond to analogous members in Fig. 1. For the sake of structural rigidity the parts of the apparatus preferably are welded together where possible.

The fact that the extended surface electrode of Gas bearing suspended particles is conducted into inlet header Ii], and passed through the tube l8 in an upwardly direction, where the particles receive a charge in the ionizing zone and are precipitated and collected in the precipitation zone.

The cleaned gas is led to the receiving header 22 and thence by means of pipe 26 to the point of destination.

A liquid wetting medium is caused to flow from feeding basin 23 in a continuous film downwardly through tube l8 into the bottom of inlet header 19, from which it is withdrawn, and if desired, cleaned and recirculated.

Broadly, this invention includes a method of removing suspended particles from gases by subjecting the gas to the action of corona or silent discharge in a high tension electric field wherein the particles are charged electrically and subsequentlypassing the gas through a second high tension electric field substantially free from corona discharge, the latter of said fields, or alternatively, each of said fields, terminating on a substantially continuous film of liquid.

An electrical precipitation apparatus for carrying out the method of this invention comprises a complementary electrode system including a discharge electrode element adapted to create an ionizing field and an opposing non-discharging electrode, a second complementary electrode system comprising spaced non-discharging electrode elements, means for directing a gas to be treated successivey between the complementary members of said electrode systems, means for impressing a high potential across the complementary electrodes of each of said electrode systems, and means for maintaining a liquid film on that one of the complementary electrodes of the second of said electrode systems,whose sign or polarity is opposite to that of the discharge electrode, or upon said electrode element and also upon the nondischarging electrode of the first complementary electrode system.

The invention is not limited to the two types of precipitators described above by way of illustration. The principles of the invention may be applied to other forms of two-stage precipitators,

including precipitators in which either or both the charging and the precipitation zones have plate collecting electrodes. units embodying the principles of the invention may be combined in a single apparatus having common gas and liquid headers for a number of ionizing and collecting .units.

Furthermore, the invention may include construction in which the electrodes of the precipitating field are not electrically or mechanically connected. to the electrodes of the charging field, as is the case in the specific embodiments shown in the drawing. Thus, the electrode system of the charging field may be entirely separate from the electrode system of the precipitating field, and the two electrode systems may be separately supported, and any electrode of either system may be insulated from any one of the other electrodes. Also, different electrical potentials, obtained either from a common source or from separate sources, may be maintained between the A plurality of tube much larger capacity than has previously been the case. This is true whether the material collected is a good conductor or a poor conductor.

Poorly conducting material tends to form an insulating layer over the electrode surface that requires a large potential drop across the layer for the electric field to penetrate to the electrode surface. Since the potential difference between the two electrodes is assumed to be constant, the potential between the high tension electrode and the surface of the deposit, which is the effective precipitation voltage, is proportionately decreased. But both forms of treater eliminate this cause of decreasing field strength in the precipitating stage by continually washing the collecting electrode surface clean and preventing vantages since the non-discharging electrode in,

the charging field (i. e., the portion of tube l8 opposite wire 24) is continually washed by the liquid film and. the precipitated particles are carried away before any harmful quantity can be deposited. Keeping the non-discharging electrode I8 clean eliminates formation of positive ions in the charging field and secures maximum degreeof negative charging of the dust particles in the gas stream. Any positive ionization reduces or may even neutralize the negative charge on the dust particles, thus reducing the precipitation efiiciency. In some cases positive ionization, if it occurs to an excessive extent, may even create positive charges on some particles which are then precipitated on the high tension electrode in the precipitating stage, where they are not removed by the liquid film.

The novel step of providing a liquid film trapping means in the precipitating zone of a two-- stageprecipitator allows operation of the apparatus at higher gas velocities than could otherwise be used were such a trapping means not provided.

The novel characteristics of my improved apparatus have unsuspected influences on the production of ozone and nitric oxide, as the precipitator operates now with only negligible formation of these undesirable products. Formation of ozone and nitric oxide occurs infields having corona discharge and the rate of formation of each gas is roughly proportional to both the current consumed and the length of discharging electrode. Since a decrease in electrode length decreases the current, it also decreases the quantity of ozone and nitric oxide formed in proportion to the second power of the change in length. Ozone is formed at a relatively constant rate independently of the gas velocity past the electrodes within the range of velocities used in practice, but of course the concentration varies inversely as the velocity so that the increased velocity at which this novel precipitator can operate further reduces the unit concentration of ozone or nitric oxide. Tests have shown that conventional single-field precipitators as now constructed and operated are usually unsatisfactory in ventilating units since they produce ozone considerably in excess of the upper limitof tolerance now adopted which is about .1 p. p. m. By comparison, the treaters above described have been found to produce only .02 to .1 p. p. m. of ozone with about .04 p. p. m. regularly obtainable under favorable conditions. Thus the ozone concentration is materially less than that produced by a single-field precipitator of equal capacity and the production of nitric oxide shows a similar decrease.

The liquid film has the particular advantage that the liquid may be used to absorb to some extent the gaseous products of the electric discharge.v Water alone, or an alkaline aqueous solution may be used to absorb oxides of nitrogen, and a solution of potassium iodide may be used to absorb the ozone. This absorbing action is increased in the form of Fig. 4 having a liquid film in the first stage'where the electric wind promotes contact of gas with the liquid on the electrode walls.

From the foregoing description it will be apparent that various other shapes and arrangements of electrodes may be used in conjuncti n with a water film without departing from the spirit and a scope of my invention, and consequently it is to be understood that the foregoing disclosure is to be considered as illustrative of, rather than restrictive upon, the appended claims.

I claim:

1. The method of removing suspended particles from gases which comprises subjecting the gas to the action of corona discharge in a high tension electric field wherein the suspended particles are charged, subsequently passing the gas through a second high tension electric field substantially free of corona discharge, and maintaining in said second field a substantially continuous film of liquid, said second field being maintained in a direction to force the charged particles toward the liquid film.

2. Electrical precipitation apparatus comprising a complementary electrode system. including a discharge electrode element adapted to create an ionizing field, a second complementary electrode system comprising spaced non-discharging electrode elements, means for directing a gas to be treated successivelybetween the complementary members of said electrode systems, means for impressing a high potential across the complementary electrodes of each of said electrode systems, and means for maintaining a liquid film on the electrode of the second of said electrode systems which is of opposite sign with respect to said discharge electrode element.

3. An electric precipitator comprising two electrode tube sections having their axes in alignment and connected electrically, means for establishing a flow of gas serially through said sections, means for flowing a' filmof liquid over the insidesurface of the electrode section which is second in the direction or gasfiow and sum! means exterior to said sections for collecting the film of liquid as it leaves said second electrode section, a central electrode disposed coaxially of the tube sections, the extreme end portion of which is of a character to bring about corona discharge in a high tension electric field, said end portion projecting into the first electrode tube section from the portion of the central electrode which is surrounded by the second electrode tube section, a hollow shield forming part of the central electrode which lies within the second electrode tube section, which shield is adapted to prevent corona discharge between the central electrode assembly and the second tube section, and means for impressing a high potential between the central electrode and said electrode tube sections.

- 4. An electric precipitator comprising a hollow tubular electrode member, gas inlet and outlet means at opposite ends of the tubular electrode, a central electrode assembly disposed coaxially of the tubular electrode, the extreme end portion of said central electrode being shaped to produce corona discharge in cooperation with a section of the tubular electrode under the influence of a high tension electric field,- the remaining portion of the central electrode surrounded by the tubular electrode being provided with a tubular shielding member which forms an integral part of the central assembly and is adapted to eliminate corona, discharge between the shielded portion of the central electrode and the surrounding portion of the tubular electrode, means for causing a film of liquid to continuously wet 'at least a portion of the inside surface of the tubular electrode, and means for impressing a high potential between the central electrode and the tubular electrode.

5. In a method of removing suspended particles from gas wherein the suspended particles are precipitated from the gas by subjecting a stream of the gas to the action of a corona discharge in a high tension electric field wherein the suspended particles are charged and subsequently passing the gas through a second field substantially free from corona discharge to cause the charged particles to move under the influence of said field to a boundary of said field, the

improvement which comprises passing the gas through the second field at a velocity sufficient to normally cause substantial re-disersion of precipitated material from a dry surface at said boundary and maintaining in said second field a substantially continuous film of liquid at said boundary thereof to intercept the charged par- -ticles moving under the influence of said field and thereby substantially prevent the occurrence of such re-dispersion.

. EVALD ANDERSON. 

